Rivet analyses

Mass spectrum for K⁻K_S⁰ decays of the τ⁻ lepton

Experiment: BABAR (PEP-II)

Inspire ID: 1679886

Status: VALIDATED

Authors: - Peter Richardson

References: - Phys.Rev. D98 (2018) no.3, 032010

Beams: * *

Beam energies: ANY

Run details: - e+ e- > tau+ tau-, or any process producing tau leptons

Mass spectra for K⁻K_S⁰ in tau decays measured by BaBar. Useful for testing models of the hadronic current in τ decays.

Source code:`BABAR_2018_I1679886.cc`

// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {


  /// @brief tau -> K+ KS nu_tau
  class BABAR_2018_I1679886 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BABAR_2018_I1679886);


    /// @name Analysis methods
    /// @{

    /// Book histograms and initialise projections before the run
    void init() {

      declare(UnstableParticles(), "UFS");
      book(_h_KK, 1, 1, 1);
      book(_v_KK, 1, 1, 2);
      book(_c, "TMP/c");
    }

    void findDecayProducts(const Particle& mother,
                           unsigned int& nstable,
                           unsigned int& nK0,
                           unsigned int& nKp,
                           unsigned int& nKm,
                           FourMomentum& ptot) {
      for (const Particle& p : mother.children()) {
        int id = p.pid();
        if (id == PID::KPLUS) {
          ++nKp;
          ++nstable;
          ptot += p.momentum();
        }
        else if (id == PID::KMINUS) {
          ++nKm;
          ++nstable;
          ptot += p.momentum();
        }
        else if (id == PID::K0S) {
          ++nK0;
          ++nstable;
          ptot += p.momentum();
        }
        else if (id == PID::PI0 || id == PID::PIPLUS || id == PID::PIMINUS) {
          ++nstable;
        }
        else if (!p.children().empty()) {
          findDecayProducts(p, nstable, nK0, nKp, nKm, ptot);
        }
        else
          ++nstable;
      }
    }

    /// Perform the per-event analysis
    void analyze(const Event& event) {

      // Loop over taus
      for (const Particle& tau : apply<UnstableParticles>(event, "UFS").particles(Cuts::abspid == PID::TAU)) {
        unsigned int nstable(0), nK0(0), nKp(0), nKm(0);
        FourMomentum p_tot(0, 0, 0, 0);
        findDecayProducts(tau, nstable, nK0, nKp, nKm, p_tot);
        if (tau.pid() < 0) {
          swap(nKp, nKm);
        }
        if (nstable != 3) continue;
        if (nKm == 1 && nK0 == 1) {
          double q = p_tot.mass();
          _h_KK->fill(q);
          double mtau = tau.mass();
          double Cq = q * sqr(sqr(mtau) - sqr(q)) * (sqr(mtau) + 2. * sqr(q));
          double fact = pow(mtau, 8) / Cq;
          _v_KK->fill(q, fact);
          _c->fill();
        }
      }
    }


    /// Normalise histograms etc., after the run
    void finalize() {
      normalize(_h_KK);
      // values from PDG 2023 for constants, see eqn 1 of paper
      double brKK = 0.5 * 1.486e-3; // PDG is K0 divide by 2 to get KS0
      double brE = 0.1782;
      double Vud = .97367;
      scale(_v_KK, 1000. * brKK / brE / sqr(Vud) / 12. / M_PI / *_c);
    }

    /// @}


    /// @name Histograms
    /// @{
    Histo1DPtr _h_KK, _v_KK;
    CounterPtr _c;
    /// @}
  };


  RIVET_DECLARE_PLUGIN(BABAR_2018_I1679886);


}

Rivet analyses

Mass spectrum for K−KS0 decays of the τ− lepton

Source code:BABAR_2018_I1679886.cc

Mass spectrum for K⁻K_S⁰ decays of the τ⁻ lepton

Source code:`BABAR_2018_I1679886.cc`